Heretofore, as an image forming apparatus such as laser printer, there has been known one having a scanner motor which rotates a beam-scanning polygon mirror. The scanner motor usually uses oil or the like fluid in its bearing portion. Since the fluid is high in temperature dependency, a time period which is required since the start of the scanner motor till the arrival thereof at a predetermined rotational speed is liable to become longer at low temperatures than at normal temperatures. At the low temperatures, therefore, notwithstanding that the scanner motor is in a normal state, it sometimes fails to arrive at a reference speed within a prescribed term, so the normal state is erroneously sensed as the fault of the scanner motor.
To cope with the erroneous sensing, JP-A-2001-83451 discloses a technique wherein a temperature detection unit for detecting the temperature of the apparatus is disposed, and subject to the judgment of a low temperature state by the temperature detection unit, a predetermined term for judging that the rotational speed of the scanner motor has risen up to a revolution number actually usable is lengthened more than in an ordinary mode in accordance with the temperature, so as to prevent the erroneous sensing of the scanner motor fault. Thus, the erroneous sensing of the scanner motor fault can be suppressed.